Aromatic alkyls are commonly catalytically oxidized, in the liquid phase and within pressurized oxidation reactors, to produce aromatic carboxylic acids. U.S. Pat. Nos. 3,092,658 and 3,170,768, both to Baldwin, are illustrative. Typically, a reaction medium within the reactor includes the aromatic alkyl, the oxidation catalyst, an oxygen-containing gas, and a solvent. The oxidation reaction is exothermic, the solvent is volatilizable and, by means of a reflux system, is used to control temperature of the reaction medium. In particular, a substantial portion of the reaction-generated heat is removed by evaporating a portion of the solvent from the reactor and withdrawing the thus-evaporated portion from the reactor as a reactor overhead vapor stream. The vapor stream is then partially condensed, and the condensate is returned to the reactor.
A product stream from the reactor contains, in addition to the produced aromatic carboxylic acid, minor amounts of catalyst, solvent, oxidation reaction by-products, and certain other process stream impurities. One process stream impurity that is present is sodium ion. The presence of sodium ions is undesirable; however, they enter process streams when certain sodium-containing compounds such as sodium hydroxide are utilized to unplug certain transfer lines or to clean the process equipment.
The reactor product stream is passed through separation equipment for producing a concentrated aromatic carboxylic acid product stream and a residue stream which contains a major portion of the catalyst, the solvent, the oxidation-reaction by-products, and the process stream impurities that had been contained in the reactor product stream. Inasmuch as the catalyst contained in such a residue stream still possesses useful catalytic activity, it is desirable to recover and return at least a portion of the residue stream-contained catalyst to the oxidation reactor. To that end, the residue stream currently is separated into a catalyst-depleted stream and one or more catalyst-containing return streams that are returned to the reactor. The return streams, at present, also contain the oxidation reaction by-products as well as the other process stream impurities mentioned above. Over a period of time, the impurity concentration in the oxidation reactor builds to a point where the impurity concentration can enhance formation rates of undesirable reaction by-products.
Conventional methods of purging the impurities from the oxidation reactor tend to cause sizable amounts of the oxidation catalyst and solvent to be purged as well. Current catalyst and solvent costs make such an impurities-purging method to be economically undesirable. Such a purging method also undesirably reduces product yields.
Substantial efforts have been heretofore expended toward improving separation of the impurities from the above-mentioned catalyst-containing return stream before such stream is returned to the reactor. However, all such known methods to effectively separate the impurities from the catalyst-containing stream prior to its return to the reactor have turned out to be, upon investigation, either technologically impractical, economically undesirable, or both.
Because of the large amounts of the various aromatic carboxylic acid products that are currently being produced, it is desirable to provide a cost-effective and technologically-feasible method to effectively separate the impurities from the catalyst-containing stream prior to its return to the reactor. The present invention provides a substantial advance toward that objective.